In view of the increasing stringency of emission control regulations in various countries in recent years, many attempts have been made to improve fuel supply systems of engines to reduce noxious exhaust emissions while maintaining good engine driveability.
In one approach to reduce noxious emissions, fuel and air are supplied to the cylinders in stoichiometric proportions and the pollutants are removed by using a catalyst. With this approach, it is desirable to control the composition of the fuel mixture to the stoichiometric proportions for all engine operating conditions. If the mixture composition departs from these proportions, this may result in a deterioration of fuel economy or an increase in the pollutants which have to be removed by the catalyst.
In another approach to reducing noxious emissions, known as the "lean burn approach", a mixture containing excess air is supplied to the cylinders. Production of pollutants in the form of carbon monoxide and oxides of nitrogen is much less than with the stoichiometric approach. Also, an arrangement using this approach is less prone to deterioration with time than an arrangement using the stoichiometric approach and this approach results in an improvement in fuel consumption in comparison with the stoichiometric approach.
With the lean burn approach, as will now be explained, the mixture composition must be controlled carefully.
The formation of oxides of nitrogen is associated with high temperatures within the combustion chamber. The highest temperatures occur with mixtures whose composition is close to stoichiometric. Under these conditions, there is little free oxygen to participate in the formation of oxides of nitrogen. Therefore, the rate of formation of oxides of nitrogen is greatest with mixtures containing some excess air. Formation of oxides of nitrogen is reduced if the peak temperature during combustion is reduced by diluting the mixture either with excess air or with exhaust gas. Thus, either the air/fuel ratio or the amount of exhaust gas recirculated must be maintained above a predetermined minimum boundary to keep the generation of oxides of nitrogen within an acceptable level.
In a combustion chamber in a spark ignition engine, immediately after a spark has occurred, no measurable combustion pressure rise occurs while a flame kernel grows from the spark to a size at which the heat release produces rapid flame propagation. This initial period of kernel growth is often termed "the delay period". The period of rapid flame propagation shall hereinafter be referred to as "the combustion period".
During the combustion period, flame propagation occurs at a finite speed. It has been found that maximum efficiency occurs when the peak pressures are generated approximately 5.degree. to 15.degree. after a piston has passed the top dead center position. In order to achieve this, it is arranged that ignition occurs before the top dead center position.
As the mixture composition is made progressively leaner, the delay period increases and flame speed falls, thereby extending the combustion period.
In a spark ignition engine, the term "spark advance angle" is used to describe the angle, before the top dead center position, at which a spark occurs.
To keep the peak pressure position near the optimum value, the spark advance must be advanced further as the mixture is made leaner. With very lean mixtures or with very high levels of exhaust gas recirculation, the delay and combustion periods are very long and the spark advance is very large. Consequently, the temperatures and pressures of the mixture at the moment of ignition are low and the rate of development of the flame kernel is also low. Small variations in the mixture composition and turbulence level can lead to large variations in the delay period which in turn leads to large variations in the total time to burn the mixture, as the combustion period can often be forced significantly into the expansion stroke of the piston.
These large variations in total burning time lead to similar variations in the cylinder pressure from cycle to cycle and so to unstable engine operation known as engine roughness. Additionally, a completely non-burning or partial burning cycle may occur where either the flame kernel does not develop, or the propagating flame is extinguished due to expansion of the cylinder volume. This leads to significant levels of emission of unburned hydrocarbons from the fuel.
Consequently, it is necessary to keep the air/fuel ratio or, where the exhaust gas is recirculated, the recirculation ratio of exhaust gas below a predetermined boundary beyond which roughness or emissions of unburned hydrocarbons become unacceptable.
Modern systems for controlling spark advance and mixture composition in an internal combustion engine make use of look-up tables stored in read only memories. These look up tables, which are also known as demand tables contain spark advance values and mixture composition values as a function of two different engine operating parameters such as engine speed and manifold pressure. These look up tables represent a considerable improvement on the mechanical devices which were used previously. However, they do not provide a completely adequate answer to emission and efficiency problems as there are many variables which they cannot take into account. These variables include changes in the accuracy of the operation of the equipment which controls the fuel mixture.
Various closed loop systems have been proposed to compensate for these variables.
In an article entitled "Electronic Spark Timing Control for Motor Vehicles" by Paul H. Schweizer and Thomas W. Collins, published by The Society of Automotive Engineers as SAE paper 780655, and also in U.S. Pat. No. 4,026,251, there is described a system for optimizing spark advance. In this system, small perturbations are superimposed on the spark advance and the resulting changes in engine speed are used to determine the differential or slope of engine speed with respect to spark advance angle. The spark advance is then adjusted until the slope is zero.
Although this system results in optimum spark advance and, consequently, optimum engine output torque, for the prevailing fuel mixture, it does not compensate for errors in mixture composition.
Another closed loop system uses an exhaust gas oxygen sensor. Unfortunately, such sensors have not proved to be accurate in use. Also, where exhaust gas is recirculated, an oxygen sensor cannot compensate for errors in the accuracy of the equipment responsible for the recirculation.